MRI-Based Use of Neoadjuvant Chemoradiotherapy in Rectal Carcinoma: Surgical Quality and Histopathological Outcome of the OCUM Trial

Background Preoperative magnetic resonance imaging (MRI) allows highly reliable imaging of the mesorectal fascia (mrMRF) and its relationship to the tumor. The prospective multicenter observational study OCUM uses these findings to indicate neoadjuvant chemoradiotherapy (nCRT) in rectal carcinoma. Methods nCRT was indicated in patients with positive mrMRF (≤ 1 mm) in cT4 and cT3 carcinomas of the lower rectal third. Results A total of 527 patients (60.2%) underwent primary total mesorectal excision, and 348 patients (39.8%) underwent long-term nCRT followed by surgery. The mrMRF was involved in 4.6% of the primary surgery group and 80.7% of the nCRT group. Rates of resections within the mesorectal plane (90.8%), sparing of pelvic nerves on both sides (97.8%), and number of regional lymph nodes (95.3% with ≥ 12 lymph nodes examined) are indicative of high-quality surgery. Resection was classified as R0 in 98.3%, the pathological circumferential resection margin (pCRM) was negative in 95.1%. Patients in the nCRT group had more advanced carcinomas with a significantly higher rate of abdominoperineal excision. Independent risk factors for pCRM positivity were advanced stage (T4), metastatic lymph nodes, resection in the muscularis propria plane, and location in the lower third. Conclusions The risk classification of rectal cancer patients by MRI seems to be highly reliable and allows the restriction of nCRT to approximately half of the patients with clinical stage II and III rectal carcinoma, provided there is a high-quality MRI diagnostic protocol, high-quality surgery, and standardized examination of the resected specimen.